Overview of Systematic Bias in Utah Snow Data
Total Page：16
File Type：pdf, Size：1020Kb
AN EXAMINATION OF EXTERNAL INFLUENCES IMBEDDED IN THE HISTORICAL SNOW DATA OF UTAH Randall P. Julander1 Michael Bricco2 ABSTRACT Snowpack data collection has a long and storied history in Utah as well as the western United States. Many researchers use historical snow course data for various applications ranging from water supply forecasting to climate change. These data are far from a perfect data set and data users should know the errors and limitations within them. In the current setting, only those collecting the data have access to the raw data and the site biographical information. In Utah, records extend back to at least 1912. Systematic measurements began in the mid 1920's with many long term snow courses established at that time. In an extremely fortuitous circumstance, Mr. George D. Clyde (former Governor of Utah) was responsible for the Snow Survey Program during the 1930's in Utah and had foresight to document each existing Snow Course in the year 1936. Each site was meticulously mapped, described and most important, photographed from several angles. Comparisons are made between the 1936 photographs, maps and descriptions and current conditions, specifically with regard to vegetation and sample point location. General conclusions are made regarding the impact that vegetation change has had on snow accumulation at each course. Changes in sampling technique and data processing are documented, particularly with regard to sample density and the re-sampling (or lack thereof in the record up to the 1950's) of individual sample points when density thresholds are exceeded. With the advent of weather modification programs, changes in snow accumulation could reasonably be expected. Utah began a relatively small test weather modification program in the 1950's in central Utah. The Utah cloud seeding act was passed in 1973 and the seeding program has continued since that time. Snow Courses affected by this program are identified and the potential impact on historical data. Finally, recommendations for individual snow course suitability for long term study based on consistency are made for each of the courses examined. SNOTEL sites, the automated version of the snow courses began to be installed in the late 1970's and early 1980's. These sites to a lesser degree due to the shorter historical time of data collection, have been impacted by vegetation change as well. They also have data impacted by sensor changes and weather modification. Because snow course data are often used to extrapolate the SNOTEL data set to a longer time frame, it is important that external influences in this data set are quantified as well. INTRODUCTION The Natural Resources Conservation Service (NRCS), United States Department of Agriculture (USDA) is charged with the task of measuring high elevation hydroclimatic data in the western United States. Snow water equivalent is the data parameter of primary interest with precipitation, temperature and other parameters also being measured. Snowpacks in Utah have been systematically measured to some extent since the mid-1920's. This is one of the best long-term databases of snow water equivalent (Soil Conservation Service 1979). These data provide a wealth of climate and runoff correlation data. The quantification of an average climatological condition as well as Paper presented Western Snow Conference 2006 1 Natural Resources Conservation Service, USDA 2 Natural Resources Conservation Service, , USDA the associated extremes has long had relevance in the categorization and characterization of various geographic regions. Recent events compared to both the historical observed data records as well as the inferred or synthetically generated geological climate record have led to a general conclusion of accelerated global climate change and a generally heightened interest in how climate at the local level may be impacted. Saunders and Maxwell state "in the most thorough review yet of changes in the West's snowpacks, an analysis of the records of 824 government snowpack-measurement sites across the West with records from 1950 to 1997 shows that snowpack levels have declined at most of those sites over that period". Cooley found in modeling studies of a Montana snowpack, that changes in temperature by 2 to 4 degrees C, could reduce snowpacks by as much as 80 percent. Gleick and Adams state that snow and ice cover are decreasing and melting earlier on average in the northern hemisphere. They state further, that field surveys show that snow cover over the northern hemisphere land surface since 1988 has been consistently below average over the last quarter century with an average decrease of about 10 percent, and that the changes have been linked to temperature. (Gleick and Adams, 2000). New research indicates that air pollution aerosols may suppress orographic precipitation by slowing down cloud drop coalescence and riming of ice precipitation, delaying the conversion of cloud water into precipitation. Mid level precipitation can be diminished by 15% to 25% of the annual total (Givati and Rosenfeld, 2004, Griffith, Solak and Yorty, 2005). As the global climate changes globally, climate changes in regional areas, such as the State of Utah, may or may not be impacted to the same degree. Thirty years is the World Meteorological Organization (WMO) standard time frame for calculating an average condition for snowpack snow water equivalent, precipitation, etc. These thirty-year averages are re-calculated and published every ten years. These climatic normals are then used to compare current observed parameters of snowpack, precipitation, to seasonal water supply and other analyses. This process then allows for a relatively consistent dataset to be used for water supply prediction. Snowpack data are being consistently used as an indicator of global warming. It is essential that individuals using snowpack data to make historical comparisons and attribute any differences to a single cause, quantify as best they can, all influences imbedded in the data. Snow data may be impacted by site physical changes, vegetation changes, weather modification, pollution, sensor changes, changes in transportation or sampling date, comparisons of snow course to SNOTEL data, changes in measurement personnel or recreational and other factors. Another factor is a sensor that does not come back to zero at the end of the snow season. In order to conclusively prove that global warming is impacting snowpacks in Utah using historical snow data, we believe that two criteria must be met: 1) current data must be significantly different than what has historically occurred using the longest period of record, and 2) other factors impacting the data must be ruled or factored out as a predominant cause, thus leaving climate change as the main or only factor. This does not imply that climate change is not happening in Utah but is simply an attempt to make sure that what is being seen in the data can be specifically attributed to a specific cause. METHODS Elements that impact the long term continuity of snow data collection in Utah were identified. A matrix of these elements was prepared for each snow data collection site. Each element was then graded based on the overall impact at that site. A range of expected impact was generated as well as an average or most likely effect on the site. These values were taken from published literature such as those documenting the impacts of vegetation on snow accumulation as well as those documenting the impacts of cloud seeding. Other impacts such as those associated with sensor change from steel to hypalon pillows were based on observed differences at sites that had both sensors simultaneously. Impacts from site physical changes, though rare were based on best estimation or simply noted as an impact and left un-quantified. Vegetation changes at snow data sites can range from none at all to complete and dramatic. Trial Lake snow course is in a small meadow and the comparison of old photos to current conditions show little to no change in the forest canopy at this site over the 80 plus years of existence. Brian Head on the other hand used to be in a mature spruce forest that has since been beetle-killed and logged. From the complete canopy of huge spruce trees to an open campground, there has been a significant change in snow accumulation and ablation characteristics. Midway Valley Snotel is in the same condition. Sites like these are noted in the analysis to have zero impact from vegetation change because the change has happened in the past several years and thus to the point of recent impact, the historical data are free from the impact but current and future data will be impacted. Likewise Little Grassy and Lily Lake have had huge vegetation changes due to wildfire and will be significantly impacted, however the current historical record is free from the influence. This analysis of vegetative change is admittedly, subjective in nature, completed by multiple Snow Survey staff members and based on comparisons with old photos compared to current conditions where possible. Where older photos were not available, the state and type of current vegetation was used. For example, is the site located in a vigorously growing stand of aspens as opposed to a site in a mature Lodgepole forest. In the case of the aspens, the assumption is that this site will have vegetative change over a period of time. In the case of the mature Lodgepole forest, minimal vegetative change is expected. An arbitrary ranking system was used to classify the vegetative change using 4 categories: 1- no change, 2- minimal change, 3- moderate change and 4- major change during the sites existence. These categories were then converted to a range of percentage change based upon documented impacts on snow accumulation from other research. In this analysis, 23 of 134 sites were ranked as having had no significant vegetation change during their existence.